This invention relates to processes and apparatus for handling masses of particulate matter. The efficient operation of such processes and apparatus has been hampered by the accumulation of static charge by the particles, particularly under conditions of low humidity. The accumulation of static charges is a function of the contact of the particles against one another and various components of the apparatus used to perform processes such as sieving, blending or dispensing of the particulate matter. Static charge accretion is disadvantageous because it causes the particles to aggregate into masses or to adhere to the surfaces of the particle processing apparatus. For the purposes herein a treatment region is defined as the area within a particle handling apparatus in which static charge will ordinarily be acquired by the particles due to their motion relative to one another on the apparatus. An example of a treatment region is a blending chamber. A treatment process is a process for handling particles during which static charge will ordinarily be acquired by the particles by their motion in a treatment region.
Applicant has observed this problem to be particularly acute when processing particles of dried animal body fluid such as human or bovine blood serum. The particles may be produced either by bulk lyophilization of sera, followed by low temperature comminution of the dried cake, or by spraying a stream of serum into a moving bath of refrigerant maintained at a temperature below the freezing point of the serum, followed by recovery and lyophilization of the frozen serum particles. The latter method has been preferred because it yields particles having more uniform geometry and mass, and it entails less loss of heat labile constituents such as enzymes.
Such serum products have numerous uses. However, one especially valuable use is in the manufacture of controls and calibrators. Since the serum is dry, large numbers of production lots may be combined into a single, large bulk lot from which quantities of dried sera may be metered into portions convenient for use in the clinical laboratory. The assembly of bulk lots has made it possible to obtain serum constituent uniformity over a much larger mass of product than has heretofore been possible. Among other advantages these lots permit a reduction in the amount of constituent assaying which must be done to provide a control having predetermined values.
Substantial difficulty has been encountered in placing this method into commercial use. The problem has centered on static build-up on the particles during such steps as sieving the production lots to eliminate particles outside a predetermined range, combining the production lots in a blender and metering the product portions into vials. The disadvantage of static charge accretion is that the final control or calibrator is rendered nonhomogeneous from vial to vial. This appears to be the result of retardation or outright removal of certain constituents during their passage through the various processing steps. For example, small particles are more likely to be removed from the product stream by static effects during blending because their light weight will result in adherence to the blender surfaces. On the other hand, static-aggregated large particles are more likely to be removed during seiving than aggregated small particles.
Further, it has been found that small serum particles, e.g. 12 mesh or smaller, exhibit depressed levels of creatine phosphokinase when compared to particles of larger size. Particle size may also differ among serum particles and additives such as drugs or additional serum constituents added to elevated normal levels. For example, these additives may be present as fine powders while the serum particles are in the form of small pellets. Additionally, these additives and different serum lots exhibit static behavior entirely independent of mass effects. For example, serum lots having different metal ion and protein levels can be expected to exhibit different static charges. The result is that the end of any given product stream will be enriched in, or possibly entirely devoid of some constituents. Thus it has been found that permitting serum particles to accumulate static charge results in considerable difficulty in securing vial to vial homogeneity.
Similar problems may be expected when processing other substances than sera. For example, blending drums for manufacture into homogeneous tablets or capsules, or blending plastic resins for manufacture into uniform articles, will be smilarly hampered by static charges acquired by the components being mixed.
Various techniques have been considered in an effort to prevent static charge accretion. For example, addition of an antistatic detergent to the product stream is precluded because such detergents may be deleterious to or affect the assay of serum constituents. Larger particles are less affected by static charge but they make precision weighing and filling of product vials extremely difficult. In the case of serum, obtaining particles having greater bulk density requires a burdensome concentration step. Further all of the foregoing options have not resulted in the desired control of static charge accretion.
Certain known electrical techniques for changing the static charge of particles, e.g. as shown in U.S. Pat. Nos. 2,896,263 and 3,864,602, require specialized apparatus in direct contact with the affected surface. This is impractical for use in such devices as blenders and in any case creates maintenance problems.
Accordingly it is an object of this invention to control, eliminate or prevent the formation of static charge on particles without adding a foreign substance to the particles or requiring a change in the desired size or composition of the particles.
It is a further object of this invention to control, eliminate or prevent the formation of static charge on dried, particulate animal body fluid.
It is another object to blend separate lots of dried, particulate animal body fluids whole maintaining the homogeneity of the blended product.
It is still another object to provide a method and apparatus for controlling, eliminating or preventing the formation of static charge in a region of a particle handling apparatus wherein such accumulation of charge normally occurs, but without requiring the presence of additional apparatus in that region.
Other objects of this invention will be apparent to those skilled in the art from a consideration of this specification taken in its entirety.